In Recent years, with portable radio terminal, folding models have become the mainstream, and, in addition, there is an increasing demand for thinner models. Moreover, portable radio terminal that support a plurality of frequency bands and portable radio terminals that support a plurality of communication systems, such as GSM system and W-CDMA system are becoming the main stream.
FIG. 1 is an overall view of a conventional folding portable radio terminal. In the folding portable radio terminal shown in FIG. 1, first housing 28 and second housing 29 are openly and closely coupled via hinge section 30. First circuit board 31 is set in the first housing 28, and antenna element 32 that resonates in a plurality of frequency bands and second circuit board 33 are set in second housing 29. In addition, radio section 34, power feeding section 35 that feeds power from radio section 34 to antenna element 32 and matching circuit section 36 that matches antenna element 32 and radio section 34 are set on second circuit board 33. Moreover, first circuit board 31 and second circuit board 33 are connected via flexible cable 37.
FIG. 2 shows VSWR (Voltage Standing Wave Ratio) of antenna element 32 in the folding portable radio terminal shown in FIG. 1 in the closed state and the open state. As shown in FIG. 2, in the closed state (the solid lines in FIG. 2), the frequency bands with VSWR equal to or lower than the threshold “3” are assured for both the first and second frequency bands. However, in the open state (the dashed lines in FIG. 2), since the length of the housing varies from the closed state and a metal body such as first circuit board 31 comes close to the antenna element, the resonance frequency having been adjusted to the lowest frequency band (first frequency band) in the closed state shifts to a lower frequency, so that the antenna gain in the desired frequency band drops.
Technologies to solve this problem are disclosed, for example, in patent documents 1 and 2. FIG. 3 is a drawing showing a conventional variable antenna matching circuit described in patent document 1. The conventional variable antenna matching circuit shown in FIG. 3 is set with impedance matching circuit 40 between antenna 38 and transmitting/receiving radio circuit 39. Capacitance element 41 and switch 42 are set in impedance matching circuit 40. Switch 42 switches between connection and disconnection of capacitance element 41 and antenna 38 based on output signals from shape detecting circuit 43 that detects shape change of the portable radio terminal.
This allows impedance matching for an antenna's impedance, which varies depending on the shape of a portable radio terminal.
FIG. 4 is a block diagram showing a circuit configuration of a variable capacity matching circuit in the conventional portable radio terminal described in patent document 2. Moreover, FIG. 5 is a drawing showing a state in which the open state and the closed state of the main body and the display section change. In the conventional portable radio terminal shown in FIG. 4, matching circuit section 48 composed of fixed capacitance 46 and variable capacitance 47 matches the impedance in helical antenna element 44 and the impedance in transmitting/receiving circuit 45. Variable capacitance 47 varies due to continuous change in opening angle α between the helical antenna element and the display section in FIG. 5.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-15307 (page 5, FIG. 2)    Patent Document 2: Japanese Patent Application Laid-Open No. 2002-353716 (page 11, FIG. 6 and FIG. 8)